 To understand why the vast majority of proton-proton collisions in our sun don't result in fusion, even though they are colliding at incredible speeds, we need to examine the strength of the electrostatic force separating them. According to Coulomb's law, two particles with the same charge will repel each other with a force that is proportional to the product of their charge and inversely proportional to the square of their distance from each other. Very much like gravitational force. In order to fuse, the protons must get close enough for the attractive strong nuclear force to take over from the repulsive electric force. The reach of the strong force is very small. Just over one centimeter or Fermi, there are a million Fermis in a nanometer. At this distance, the electric repulsion force is overwhelming. We see that the force is quite extreme, given that it is working on a mass as small as a single proton. If we look at it from a classical energy point of view, we see that proton-average energy at 15 million Kelvin is just not enough to overcome the potential energy barrier. In fact, the energy required to overcome the barrier is 300 times greater than the average energy of the protons. To understand how often we can expect a proton to have the barrier's energy at the sun's temperature, we use probability distributions developed by James Maxwell and Ludwig Boltzmann in the mid-1800s. This analysis shows that only one out of ten to the two hundredths collisions would cross the Coulomb barrier. That's almost as good as none. Our sun would simply not burn hydrogen, if this was all there was. But we know there is more, because we have measured the fusion rate of a proton in our sun's core, and it's 180 orders of magnitude more than this classical physics predicts.